A widely used and successful commercial process for synthesizing acetic acid involves the catalyzed carbonylation of methanol with carbon monoxide. The catalysis contains rhodium and/or iridium and a halogen promoter, typically methyl iodide. The reaction is conducted by continuously bubbling carbon monoxide through a liquid reaction medium in which the catalyst is dissolved. The reaction medium also comprises methyl acetate, water, methyl iodide and the catalyst. Conventional commercial processes for carbonylation of methanol include those described in U.S. Pat. Nos. 3,769,329, 5,001,259, 5,026,908, and 5,144,068, the entire contents and disclosures of which are hereby incorporated by reference. Another conventional methanol carbonylation process includes the Cativa™ process, which is discussed in Jones, J. H. (2002), “The Cativa™ Process for the Manufacture of Acetic Acid,” Platinum Metals Review, 44 (3): 94-105, the entire content and disclosure of which is hereby incorporated by reference.
During the methanol carbonylation reaction, by-product gases build up in the reactor as the crude acetic acid product is withdrawn into a flasher. The build-up of these gases in the reactor is often controlled by venting an off-gas from the reactor to keep the carbon monoxide partial pressure in the reactor at an acceptable level to maximize catalyst activity and stability. The vented off-gas may comprise carbon monoxide, inert and by-product gases, volatile halogen promoters, acetic acid, water, unreacted methanol, and/or methyl acetate. In most methanol carbonylation processes the off-gas is processed in one or more recovery units to recover volatile halogen promoters, acetic acid, water, unreacted methanol, and/or methyl acetate and return those recovered compounds to the reactor. The gases that pass out of the recovery units may be purged and/or directed to a flasher vessel to enhance catalyst stability. An example of a recovery unit is described in U.S. Pub. Nos. 2008/0293996 and 2009/0270651, the entire contents and disclosure of which are hereby incorporated by reference.
U.S. Pat. No. 5,917,089 discloses that an “off-gas” from the reactor may be fed directly to a second reactor, along with fresh methanol, to produce additional carbonylation product, i.e. acetic acid. The off-gas, as known in the art, however, is not a derivative stream.
A purification section processes the crude acetic acid product from the reactor to remove impurities thus providing a high quality acetic acid product. These impurities, which may be present in trace amounts, affect the quality of acetic acid, especially as the impurities are circulated through the reaction process, which, among other things, can result in the build up over time of these impurities. Conventional purification techniques to remove these impurities include treating the acetic acid product streams with oxidizers such as ozone, reducers such as hydrogen, water, methanol, activated-carbon, amines, and the like. The treatments may or may not be combined with the distillation of the crude acetic acid product. Typically, during the purification, there are several vents, which purge non-condensable gases formed in the reactor. The vented gases may be processed in a recovery unit to recover light boiling point components, such as the halogen promoter, as described in U.S. Pub. No. 2008/0293966, the entire content and disclosure of which is hereby incorporated by reference. The vented gases, which also contain carbon monoxide, that pass through the recovery unit are typically purged or flared. The loss of the carbon monoxide represents a loss of the reactants.
An alternative to the liquid-phase methanol carbonylation process is described in U.S. Pat. No. 6,617,471, the entire contents and disclosures of which are hereby incorporated by reference. U.S. Pat. No. 6,617,471 discloses a vapor-phase carbonylation method for producing esters and carboxylic acids from reactants comprising lower alkyl alcohols, lower alkyl alcohol generating compounds, and mixtures thereof. The method includes contacting the reactants and carbon monoxide in a carbonylation zone of a carbonylation reactor under vapor-phase conditions with a catalyst having a catalytically effective amount of iridium and tin associated with a solid carrier material.
In view of these references, the need exists for a process for processing vented gases during the purification and separation of the crude acetic acid product to recover reactants and improve the efficiency of the acetic acid production.